Hiroki Kotaka

Rashba Effect on the Structure of the Bi One-Bilayer Film: Fully Relativistic First-Principles Calculation

Using first-principles calculations, we study the spin–orbit interactions and spin textures of a Bi one-bilayer film, which attracts scientific interest because of the topological insulator and so on. The substrate effect is successfully mimicked by applying on electric field in the perpendicular direction of the film, which breaks the inversion symmetry. We study the highest occupied band around the Γ point. Although the vortex of the in-plane spin component is well explained on the basis of the conventional Rashba effect, we find a substantial out-of-plane component which cannot be explained by the conventional Rashba model. This spin texture is similar to that of a multi-bilayer Bi film, which has recently been observed using a spin-resolved angle-resolved photoemission spectroscopy experiment. We also find a spin vortex around the K point although this point has no time-reversal symmetry. We expect that a similar vortex appears in materials having the p3m1 symmetry, whose spin–orbit interactions have recently attracted scientific interest.

Edge States of Bi Nanoribbons on Bi Substrates: First-Principles Density Functional Study

By using fully relativistic first-principles calculations, we study edge states of the Bi(001) nanoribbons. We find that freestanding zigzag bismuth nanoribbons (ZBNRs) have two spin degenerate bands around the Fermi energy, whose wave functions are localized at the edges. The wave functions are sharply localized at the edges at the zone boundary and become delocalized as the wave number decreases. In the case of the ZBNR on Bi substrates, the inversion symmetry is broken. As a result, the spin degenerate bands split and thus the density of states near the Fermi level has broad distributions; therefore, the electronic structures are expected to be stabilized. Because of the edge state near the Fermi energy, conduction along the edge lines is expected. However, the topological insulator predicted in the case of the freestanding ZBNR is not achieved in the case of the ZBNR on Bi substrates.

Tunable Rashba effect on strained ZnO: First-principles density-functional study

We investigated the Rashba effect on the conduction band of strained ZnO using first-principles calculations. We found that the Rashba spin rotations can be inversed by applying biaxial strain. This rotation inversion is due to the biaxial strain changing the direction of the electric polarization around the Zn atom. We also found that the amount of Rashba splitting can be controlled by tuning the strain. These findings suggest that the strained ZnO is suitable for spintronics applications.

Polarity tuning of spin-orbit-induced spin splitting in two-dimensional transition metal dichalcogenides

The established spin splitting in monolayer (ML) of transition metal dichalcogenides (TMDs) that is caused by inversion symmetry breaking is dictated by mirror symmetry operations to exhibit fully out-of-plane direction of spin polarization. Through first-principles density functional theory calculations, we show that polarity-induced mirror symmetry breaking leads to new sizable spin splitting having in-plane spin polarization. These splittings are effectively controlled by tuning the polarity using biaxial strain. Furthermore, the admixtures of the out-of-plane and in-plane spin-polarized states in the strained polar systems are identified, which is expected to influence the spin relaxation through the Dyakonov-Perel mechanism. Our study clarified that the polarity-induced mirror symmetry breaking plays an important role in controlling the spin splitting and spin relaxation in the TMDs ML, which is useful for designing future spintronic devices.

First-principles study of Rashba effect in the (LaAlO3)2/(SrTiO3)2

We have performed first-principles density functional calculations for superlattice composed of two oxide insulators, (LaAlO3)2/(SrTiO3)2. Calculated band structures show interface-induced metallic states. We found electron doping in the TiO2 layer at LaO interface and hole doping in the AlO2 layer at SrO interface. Due to the built-in electric field at the interface, the interface states show spin splitting and vortex-like spin textures in momentum space, i.e. Rashba effect. Calculated Rashba coefficient αR, 12.6 meV Å, is of the same order as that of the experimental value.

First-principles study of surface states in topological insulators Bi2Te3 and Bi2Se3: film thickness dependence

Using first-principles method, we investigated the electronic states of Bi2Te3 and Bi2Se3. We showed that both Bi2Te3 and Bi2Se3 are insulators with a bulk band gap. In contrast, the surface states of Bi2Te3 and Bi2Se3 films have a metallic band connecting the conduction and valence bands. The films have an energy gap at the point when the film thickness is less than four quintuple layers (QLs), or about 30 Å. The energy gaps are closed at six QLs and four QLs for Bi2Te3 and Bi2Se3, respectively. We confirmed the metallicity up to nine QLs. Furthermore, we investigated the spin structures of nine-QL films at the Fermi energy in momentum space. We found that both the Bi2Te3 and Bi2Se3 films have Rashba-type spin textures; i.e. the surface states have spin–polarisation. To investigate the spatial distribution of the spin, we decomposed the expected values of the spin for each atom. The expected values of the spin are localised within the third QL from the surface. Our results of nine-QL films clearly show the boundary between the bulk and surface regions.

First-principles study of Rashba effect in ultra-thin bismuth surface alloys

Abstract We performed density functional calculations for ultra-thin bismuth surface alloys: surface alloys of bismuth and face-centered cubic metals Bi/ M (111)-( 3 × 3 ) R 30° ( M =Cu, Ag, Au, Ni, Co, and Fe). Our calculated Rashba parameters for the Bi/Ag are consistent with the previous experimental and theoretical results. We predicted a trend in the Rashba coefficients α R of bands around the Fermi energy for noble metals as follows: Bi/Ag > Bi/Cu > Bi/Au. As for the transition metals, there is a trend in α R : Bi/Ni > Bi/Co > Bi/Fe. Our finding may lead to design efficient spin-charge conversion materials.

Spin-Orbit Interaction Effects in the Electronic Structure of B20-type CoSi: First-Principles Density Functional Study

We have performed fully relativistic first-principles density functional calculations for non-magnetic B20-type CoSi. The spin-orbit interaction has crucial effects on the electronic structures of a chiral crystal. The calculated band structure around the Fermi energy shows Bloch vector

First-principles study of spin texture and Fermi lines in Bi(111) multi-layer nanofilm

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Strong Rashba effect in the localized impurity states of halogen-doped monolayer PtSe2

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